Method for compressing a higher order ambisonics (hoa) signal, method for decompressing a compressed hoa signal, apparatus for compressing a hoa signal, and apparatus for decompressing a compressed hoa signal

ABSTRACT

A method for compressing a HOA signal being an input HOA representation with input time frames (C(k)) of HOA coefficient sequences comprises spatial HOA encoding of the input time frames and subsequent perceptual encoding and source encoding. Each input time frame is decomposed ( 802 ) into a frame of predominant sound signals (X PS (k−1)) and a frame of an ambient HOA component (C AMB (k−1)). The ambient HOA component (C ˜   AMB (k−1)) comprises, in a layered mode, first HOA coefficient sequences of the input HOA representation (c n (k−1)) in lower positions and second HOA coefficient sequences (c AMB,n (k−1)) in remaining higher positions. The second HOA coefficient sequences are part of an HOA representation of a residual between the input HOA representation and the HOA representation of the predominant sound signals.

FIELD OF THE INVENTION

This invention relates to a method for compressing a Higher OrderAmbisonics (HOA) signal, a method for decompressing a compressed HOAsignal, an apparatus for compressing a HOA signal, and an apparatus fordecompressing a compressed HOA signal.

BACKGROUND

Higher Order Ambisonics (HOA) offers a possibility to representthree-dimensional sound. Other known techniques are wave field synthesis(WFS) or channel based approaches like 22.2. In contrast to channelbased methods, however, the HOA representation offers the advantage ofbeing independent of a specific loudspeaker set-up. This flexibility,however, is at the expense of a decoding process which is required forthe playback of the HOA representation on a particular loudspeakerset-up. Compared to the WFS approach, where the number of requiredloudspeakers is usually very large, HOA may also be rendered to set-upsconsisting of only few loudspeakers. A further advantage of HOA is thatthe same representation can also be employed without any modificationfor binaural rendering to head-phones.

HOA is based on the representation of the so-called spatial density ofcomplex harmonic plane wave amplitudes by a truncated SphericalHarmonics (SH) expansion. Each expansion coefficient is a function ofangular frequency, which can be equivalently represented by a timedomain function. Hence, without loss of generality, the complete HOAsound field representation actually can be assumed to consist of 0 timedomain functions, where 0 denotes the number of expansion coefficients.These time domain functions will be equivalently referred to as HOAcoefficient sequences or as HOA channels in the following. Usually, aspherical coordinate system is used where the x axis points to thefrontal position, the y axis points to the left, and the z axis pointsto the top. A position in space x=(r, θ, φ)^(T) is represented by aradius r>0 (i.e. the distance to the coordinate origin), an inclinationangle θε[0, π] measured from the polar axis z and an azimuth angleφε[0,2π[measured counter-clockwise in the x-y plane from the x axis.Further, (•)^(T) denotes the transposition.

A more detailed description of the HOA coding is provided in thefollowing.

The Fourier transform of the sound pressure with respect to time denotedby

_(t)(•), i.e., P(ω,x)=

_(t)(p(t,x))=∫_(-∞) ^(∞)p(t,x)e^(−iωt)dt with ω denoting the angularfrequency and i indicating the imaginary unit, may be expanded into theseries of Spherical Harmonics according to P (ω=kc_(s), r, θ,φ))=Σ_(n=0) ^(N)Σ_(m=-n) ^(n)A_(n) ^(m)(k)j_(n)(kr)S_(n) ^(m) (θ, φ).

Here c_(s) denotes the speed of sound and k denotes the angularwavenumber, which is related to the angular frequency ω by

$k = {\frac{\omega}{c_{s}}.}$

Further, j_(n)(•) denote the spherical Bessel functions of the firstkind and S_(n) ^(m)(θ,φ) denote the real valued Spherical Harmonics oforder n and degree m. The expansion coefficients A_(n) ^(m)(k) onlydepend on the angular wavenumber k. Note that it has been implicitlyassumed that sound pressure is spatially band-limited. Thus, the seriesis truncated with respect to the order index n at an upper limit N,which is called the order of the HOA representation. If the sound fieldis represented by a superposition of an infinite number of harmonicplane waves of different angular frequencies a) and arriving from allpossible directions specified by the angle tuple (θ,φ), the respectiveplane wave complex amplitude function C(ω, θ, φ) can be expressed by thefollowing Spherical Harmonics expansion:

C(ω=kc _(s),θ,φ)=Σ_(n=0) ^(n)Σ_(m=-n) ^(n) C _(n) ^(m)(k)S _(n)^(m)(θ,φ),

where the expansion coefficients C_(n) ^(m)(k) are related to theexpansion coefficients A_(n) ^(m)(k) by A_(n) ^(m)(k)=i^(n)C_(n)^(m)(k).

Assuming the individual coefficients C_(n) ^(m)(ω=kc_(s)) to befunctions of the angular frequency ω, the application of the inverseFourier transform (denoted by

⁻¹(•)) provides time domain functions

${c_{n}^{m}(t)} = {{\mathcal{F}_{t}^{- 1}( {C_{n}^{m}( {\omega/c_{s}} )} )} = {\frac{1}{2\pi}{\int_{- \infty}^{\infty}{{C_{n}^{m}( \frac{\omega}{c_{s}} )}^{{\omega}t}\ {\omega}}}}}$

for each order n and degree m, which can be collected in a single vectorc(t) by c(t)=[c₀ ⁰(t) c₁ ⁻¹(t) c₁ ⁰(t) c₁ ¹ (t) c₂ ⁻²(t) c₂ ⁻¹(t) c₂⁰(t) . . . c_(N) ^(N−1)(t) c_(N) ^(N)(t)]^(T). The position index of atime domain function c_(n) ^(m)T(t) within the vector c(t) is given byn(n+1)+1+m. The overall number of elements in the vector c(t) is givenby 0=(N+1)². The discrete-time versions of the functions c_(n) ^(m)T(t)are referred to as Ambisonic coefficient sequences. A frame-based HOArepresentation is obtained by dividing all of these sequences intoframes C(k) of length B and frame index k as follows:

C(k):=[c((kB+1)T _(s))c((kB+2)T _(s)) . . . c((kB+B)T _(s))],

where T_(s) denotes the sampling period. The frame C(k) itself can thenbe represented as a composition of its individual rows c_(i)(k), i=1, .. . , 0, as

${C(k)} = \begin{bmatrix}{c_{1}(k)} \\{c_{2}(k)} \\\vdots \\{c_{O}(k)}\end{bmatrix}$

with c_(i)(k) denoting the frame of the Ambisonic coefficient sequencewith position index i. The spatial resolution of the HOA representationimproves with a growing maximum order N of the expansion. Unfortunately,the number of expansion coefficients 0 grows quadratically with theorder N, in particular 0=(N+1)². For example, typical HOArepresentations using order N=4 require 0=25 HOA (expansion)coefficients. According to these considerations, the total bit rate forthe transmission of HOA representation, given a desired single-channelsampling rate f_(s) and the number of bits N_(b) per sample, isdetermined by 0·f_(s) N_(b). Consequently, transmitting a HOArepresentation of order N=4 with a sampling rate of f_(s)=48 kHzemploying N_(b)=16 bits per sample results in a bit rate of 19.2MBits/s, which is very high for many practical applications, as e.g.streaming. Thus, compression of HOA representations is highly desirable.Previously, the compression of HOA sound field representations wasproposed in the European Patent applications EP2743922A, EP2665208A andEP2800401A. These approaches have in common that they perform a soundfield analysis and decompose the given HOA representation into adirectional and a residual ambient component.

The final compressed representation is assumed to comprise, on the onehand, a number of quantized signals, which result from the perceptualcoding of the directional signals, and relevant coefficient sequences ofthe ambient HOA component. On the other hand, it is assumed to compriseadditional side information related to the quantized signals, which isnecessary for the reconstruction of the HOA representation from itscompressed version.

Further, a similar method is described in ISO/IEC JTC1/SC29/WG11 N14264(Working draft 1-HOA text of MPEG-H 3D audio, January 2014, San Jose),where the directional component is extended to a so-called predominantsound component. As the directional component, the predominant soundcomponent is assumed to be partly represented by directional signals,i.e. monaural signals with a corresponding direction from which they areassumed to impinge on the listener, together with some predictionparameters to predict portions of the original HOA representation fromthe directional signals. Additionally, the predominant sound componentis supposed to be represented by so-called vector based signals, meaningmonaural signals with a corresponding vector which defines thedirectional distribution of the vector based signals. The knowncompressed HOA representation consists of I quantized monaural signalsand some additional side information, wherein a fixed number O_(MIN) outof these I quantized monaural signals represent a spatially transformedversion of the first O_(MIN) coefficient sequences of the ambient HOAcomponent C_(AMB)(k−2). The type of the remaining I−O_(MIN) signals canvary between successive frames, and be either directional, vector based,empty or representing an additional coefficient sequence of the ambientHOA component C_(AMB)(k−2).

A known method for compressing a HOA signal representation with inputtime frames (C(k)) of HOA coefficient sequences includes spatial HOAencoding of the input time frames and subsequent perceptual encoding andsource encoding. The spatial HOA encoding, as shown in FIG. 1 a),comprises performing Direction and Vector Estimation processing of theHOA signal in a Direction and Vector Estimation block 101, wherein datacomprising first tuple sets

_(DIR)(k) for directional signals and second tuple sets

_(VEC)(k) for vector based signals are obtained. Each of the first tuplesets comprises an index of a directional signal and a respectivequantized direction, and each of the second tuple sets comprising anindex of a vector based signal and a vector defining the directionaldistribution of the signals. A next step is decomposing 103 each inputtime frame of the HOA coefficient sequences into a frame of a pluralityof predominant sound signals X_(PS)(k−1) and a frame of an ambient HOAcomponent C_(AMB)(k−1), wherein the predominant sound signalsX_(PS)(k−1) comprise said directional sound signals and said vectorbased sound signals. The decomposing further provides predictionparameters ξ(k−1) and a target assignment vector v_(A,T)(k−1). Theprediction parameters ξ(k−1) describe how to predict portions of the HOAsignal representation from the directional signals within thepredominant sound signals X_(PS)(k−1) so as to enrich predominant soundHOA components, and the target assignment vector v_(A,T)(k−1) containsinformation about how to assign the predominant sound signals to a givennumber I of channels. The ambient HOA component C_(AMB)(k−1) is modified104 according to the information provided by the target assignmentvector v_(A,T)(k−1), wherein it is determined which coefficientsequences of the ambient HOA component are to be transmitted in thegiven number I of channels, depending on how many channels are occupiedby predominant sound signals. A modified ambient HOA componentC_(M,A)(k−2) and a temporally predicted modified ambient HOA componentC_(P,M,A)(k−1) are obtained. Also a final assignment vector v_(A)(k−2)is obtained from information in the target assignment vectorv_(A,T)(k−1). The predominant sound signals X_(PS)(k−1) obtained fromthe decomposing, and the determined coefficient sequences of themodified ambient HOA component C_(M,A)(k−2) and of the temporallypredicted modified ambient HOA component C_(P,M,A)(k−1) are assigned tothe given number of channels, using the information provided by thefinal assignment vector v_(A)(k−2), wherein transport signalsy_(i)(k−2), i=1, . . . , I and predicted transport signals y_(P,i)(k−2), i=1, . . . , I are obtained. Then, gain control (ornormalization) is performed on the transport signals y_(i)(k−2) and thepredicted transport signals y_(P,i)(k−2), wherein gain modifiedtransport signals z_(i)(k−2), exponents e_(i)(k−2) and exception flags(β_(i)(k−2) are obtained.

As shown in FIG. 1 b), the perceptual encoding and source encodingcomprises perceptual coding of the gain modified transport signalsz_(i)(k−2), wherein perceptually encoded transport signals {hacek over(z)}_(i)(k−2), i=1, . . . , I are obtained, encoding side informationcomprising said exponents e_(i)(k−2) and exception flags β_(i)(k−2), thefirst and second tuple sets

_(DIR)(k),

_(VEC)(k), the prediction parameters ξ(k−1) and the final assignmentvector v_(A)(k−2), and encoded side information {hacek over (Γ)}(k−2) isobtained. Finally, the perceptually encoded transport signals {hacekover (z)}_(i)(k−2) and the encoded side information are multiplexed intoa bitstream.

SUMMARY OF THE INVENTION

One drawback of the proposed HOA compression method is that it providesa monolithic (i.e. non-scalable) compressed HOA representation. Forcertain applications, like broad-casting or internet streaming, it ishowever desirable to be able to split the compressed representation intoa low quality base layer (BL) and a high quality enhancement layer (EL).The base layer is supposed to provide a low quality compressed versionof the HOA representation, which can be decoded independently of theenhancement layer. Such a BL should typically be highly robust againsttransmission errors, and be transmitted at a low data rate in order toguarantee a certain minimum quality of the decompressed HOArepresentation even under bad transmission conditions. The EL containsadditional information to improve the quality of the decompressed HOArepresentation.

The present invention provides a solution for modifying existing HOAcompression methods so as to be able to provide a compressedrepresentation that comprises a (low quality) base layer and a (highquality) enhancement layer. Further, the present invention provides asolution for modifying existing HOA decompression methods so as to beable to decode a compressed representation that comprises at least a lowquality base layer that is compressed according to the invention.

One improvement relates to obtaining a self-contained (low quality) baselayer. According to the invention, the O_(MIN) channels that aresupposed to contain a spatially transformed version of the (without lossof generality) first O_(MIN) coefficient sequences of the ambient HOAcomponent C_(AMB)(k−2) are used as the base layer. An advantage ofselecting the first O_(MIN) channels for forming a base layer is theirtime-invariant type. However, conventionally the respective signals lackany predominant sound components, which are essential for the soundscene. This is also clear from the conventional computation of theambient HOA component C_(AMB)(k−1), which is carried out by subtractionof the predominant sound HOA representation C_(PS)(k−1) from theoriginal HOA representation C(k−1) according to

C _(AMB)(k−1)=C(k−1)−C _(PS)(k−1)  (1)

Therefore, one improvement of the invention relates to the addition ofsuch predominant sound components. According to the invention, asolution to this problem is the inclusion of predominant soundcomponents at a low spatial resolution into the base layer. For thispurpose, the ambient HOA component C_(AMB)(k−1) that is output by a HOADecomposition processing in the spatial HOA encoder according to theinvention is replaced by a modified version thereof. The modifiedambient HOA component comprises in the first O_(MIN) coefficientsequences, which are supposed to be always transmitted in a spatiallytransformed form, the coefficient sequences of the original HOAcomponent. This improvement of the HOA Decomposition processing can beseen as an initial operation for making the HOA compression work in alayered mode (for example dual layer mode). This mode provides e.g. twobit streams, or a single bit stream that can be split up into a baselayer and an enhancement layer. Using or not using this mode issignalized by a mode indication bit (e.g. a single bit) in access unitsof the total bit stream.

In one embodiment, the base layer bit stream {hacek over(B)}_(BASE)(k−2) only includes the perceptually encoded signals {hacekover (z)}_(i)(k−2), i=1, . . . , O_(MIN), and the corresponding codedgain control side information, which consists of the exponentse_(i)(k−2) and the exception flags β_(i)(k−2), i=1, . . . , O_(MIN). Theremaining perceptually encoded signals {hacek over (z)}_(i)(k−2),i=O_(MIN)+1, . . . , O and the encoded remaining side information areincluded into the enhancement layer bit stream. In one embodiment, thebase layer bit stream {hacek over (B)}_(BASE)(k−2) and the enhancementlayer bit stream {hacek over (B)}_(ENH)(k−2) are then jointlytransmitted instead of the former total bit stream {hacek over(B)}(k−2).

A method for compressing a Higher Order Ambisonics (HOA) signalrepresentation having time frames of HOA coefficient sequences isdisclosed in claim 1. An apparatus for compressing a Higher OrderAmbisonics (HOA) signal representation having time frames of HOAcoefficient sequences is disclosed in claim 3.

A method for decompressing a Higher Order Ambisonics (HOA) signalrepresentation having time frames of HOA coefficient sequences isdisclosed in claim 2. An apparatus for decompressing a Higher OrderAmbisonics (HOA) signal representation having time frames of HOAcoefficient sequences is disclosed in claim 4.

A non-transitory computer readable storage medium having executableinstructions to cause a computer to perform a method for compressing aHigher Order Ambisonics (HOA) signal representation having time framesof HOA coefficient sequences is disclosed in claim 5.

A non-transitory computer readable storage medium having executableinstructions to cause a computer to perform a method for decompressing aHigher Order Ambisonics (HOA) signal representation having time framesof HOA coefficient sequences is disclosed in claim 6.

Advantageous embodiments of the invention are disclosed in the dependentclaims, the following description and the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are described with reference tothe accompanying drawings, which show in

FIG. 1 the structure of a conventional architecture of a HOA compressor;

FIG. 2 the structure of a conventional architecture of a HOAdecompressor;

FIG. 3 the structure of an architecture of a spatial HOA encoding andperceptual encoding portion of a HOA compressor according to oneembodiment of the invention;

FIG. 4 the structure of an architecture of a source coder portion of aHOA compressor according to one embodiment of the invention;

FIG. 5 the structure of an architecture of a perceptual decoding andsource decoding portion of a HOA decompressor according to oneembodiment of the invention;

FIG. 6 the structure of an architecture of a spatial HOA decodingportion of a HOA decompressor according to one embodiment of theinvention;

FIG. 7 transformation of frames from ambient HOA signals to modifiedambient HOA signals,

FIG. 8 a flow-chart of a method for compressing a HOA signal;

FIG. 9 a flow-chart of a method for decompressing a compressed HOAsignal; and

FIG. 10 details of parts of an architecture of a spatial HOA decodingportion of a HOA decompressor according to one embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

For easier understanding, prior art solutions in FIG. 1 and FIG. 2 arerecapitulated in the following.

FIG. 1 shows the structure of a conventional architecture of a HOAcompressor. In a method described in [4], the directional component isextended to a so-called predominant sound component. As the directionalcomponent, the predominant sound component is assumed to be partlyrepresented by directional signals, meaning monaural signals with acorresponding direction from which they are assumed to impinge on thelistener, together with some prediction parameters to predict portionsof the original HOA representation from the directional signals.Additionally, the predominant sound component is supposed to berepresented by so-called vector based signals, meaning monaural signalswith a corresponding vector which defines the directional distributionof the vector based signals. The overall architecture of the HOAcompressor proposed in [4] is illustrated in FIG. 1. It can besubdivided into a spatial HOA encoding part depicted in FIG. 1a and aperceptual and source encoding part depicted in FIG. 1b . The spatialHOA encoder provides a first compressed HOA representation consisting ofI signals together with side information describing how to create an HOArepresentation thereof. In the perceptual and side info source coder thementioned I signals are perceptually encoded and the side information issubjected to source encoding, before multiplexing the two codedrepresentations.

Conventionally, the spatial encoding works as follows.

In a first step, the k-th frame C(k) of the original HOA representationis input to a Direction and Vector Estimation processing block, whichprovides the tuple sets

_(DIR)(k) and

_(VEC)(k). The tuple set

_(DIR)(k) consists of tuples of which the first element denotes theindex of a directional signal and of which the second element denotesthe respective quantized direction. The tuple set

_(VEC)(k) consists of tuples of which the first element indicates theindex of a vector based signal and of which the second element denotesthe vector defining the directional distribution of the signals, i.e.how the HOA representation of the vector based signal is computed.

Using both tuple sets

_(DIR)(k) and

_(VEC)(k), the initial HOA frame C(k) is decomposed in the HOADecomposition into the frame X_(PS)(k−1) of all predominant sound (i.e.directional and vector based) signals and the frame C_(AMB)(k−1) of theambient HOA component. Note the delay of one frame, respectively, whichis due to overlap add processing in order to avoid blocking artifacts.Furthermore, the HOA Decomposition is assumed to output some predictionparameters ζ(k−1) describing how to predict portions of the original HOArepresentation from the directional signals in order to enrich thepredominant sound HOA component. Additionally, a target assignmentvector v_(A,T)(k−1) containing information about the assignment ofpredominant sound signals, which were determined in the HOADecomposition processing block, to the I available channels is provided.The affected channels can be assumed to be occupied, meaning they arenot available to transport any coefficient sequences of the ambient HOAcomponent in the respective time frame.

In the Ambient Component Modification processing block, the frameC_(AMB)(k−1) of the ambient HOA component is modified according to theinformation provided by the target assignment vector v_(A,T)(k−1). Inparticular, it is determined which coefficient sequences of the ambientHOA component are to be transmitted in the given I channels, depending,amongst other aspects, on the information (contained in the targetassignment vector v_(A,T)(k−1)) about which channels are available andnot already occupied by predominant sound signals. Additionally, a fadein and out of coefficient sequences is performed if the indices of thechosen coefficient sequences vary between successive frames.

Furthermore, it is assumed that the first O_(MIN) coefficient sequencesof the ambient HOA component C_(AMB)(k−2) are always chosen to beperceptually coded and to be transmitted, where O_(MIN)=(N_(MIN)+1)²with N_(MIN)≦N being typically a smaller order than that of the originalHOA representation. In order to de-correlate these HOA coefficientsequences, it is proposed to transform them to directional signals (i.e.general plane wave functions) impinging from some predefined directionsΩ_(MIN,d), d=1, . . . O_(MIN). Along with the modified ambient HOAcomponent C_(M,A)(k−1), a temporally predicted modified ambient HOAcomponent C_(P,M,A)(k−1) is computed to be later used in the GainControl processing block in order to allow a reasonable look ahead.

The information about the modification of the ambient HOA component isdirectly related to the assignment of all possible types of signals tothe available channels. The final information about the assignment iscontained in the final assignment vector v_(A)(k−2). In order to computethis vector, information contained in the target assignment vectorv_(A,T)(k−1) is exploited.

The Channel Assignment assigns with the information provided by theassignment vector v_(A)(k−2) the appropriate signals contained inX_(PS)(k−2) and that contained in C_(M,A)(k−2) to the I availablechannels, yielding the signals y_(i)(k−2), i=1, . . . , I. Further,appropriate signals contained in X_(PS)(k−1) and that in C_(P,AMB)(k−1)are also assigned to the I available channels, yielding the predictedsignals y_(P,i)(k−2), i=1, . . . , I. Each of the signals y_(i)(k−2),i=1, . . . , I, is finally processed by a Gain Control, where the signalgain is smoothly modified to achieve a value range that is suitable forthe perceptual encoders. The predicted signal frames y_(P,i)(k−2), i=1,. . . ,I, allow a kind of look ahead in order to avoid severe gainchanges between successive blocks. The gain modifications are assumed tobe reverted in the spatial decoder with the gain control sideinformation, consisting of the exponents e_(i)(k−2) and the exceptionflags β_(i)(k−2), i=1, . . . , I.

FIG. 2 shows the structure of a conventional architecture of a HOAdecompressor, as proposed in [4]. Conventionally, HOA decompressionconsists of the counterparts of the HOA compressor components, which areobviously arranged in reverse order. It can be subdivided into aperceptual and source decoding part depicted in FIG. 2a ) and a spatialHOA decoding part depicted in FIG. 2b ).

In the perceptual and side info source decoder, the bit stream is firstde-multiplexed into the perceptually coded representation of the Isignals and into the coded side information describing how to create anHOA representation thereof. Successively, a perceptual decoding of the Isignals and a decoding of the side information is performed. Then, thespatial HOA decoder creates from the I signals and the side informationthe reconstructed HOA representation.

Conventionally, spatial HOA decoding works as follows.

In the spatial HOA decoder, each of the perceptually decoded signals{circumflex over (z)}_(i)(k), iε{1, . . . , I}, is first input to anInverse Gain Control processing block together with the associated gaincorrection exponent e_(i)(k) and gain correction exception flagβ_(i)(k). The i-th Inverse Gain Control processing provides a gaincorrected signal frame ŷ_(i)(k).

All of the I gain corrected signal frames ŷ_(i)(k), iε{1, . . . , I},are passed together with the assignment vector v_(AMB,ASSIGN) (k) andthe tuple sets

_(DIR)(k+1) and

_(VEC)(k+1) to the Channel Reassignment. The tuple sets

_(DIR)(k+1) and

_(VEC)(k+1) are defined above (for spatial HOA encoding), and theassignment vector v_(AMS,ASSIGN)(k) consists of I components, whichindicate for each transmission channel if and which coefficient sequenceof the ambient HOA component it contains. In the Channel Reassignmentthe gain corrected signal frames ŷ_(i)(k) are redistributed toreconstruct the frame {circumflex over (X)}_(PS) (k) of all predominantsound signals (i.e., all directional and vector based signals) and theframe C_(I,AMB)(k) of an intermediate representation of the ambient HOAcomponent. Additionally, the set

_(AMB,ACT) (k) of indices of coefficient sequences of the ambient HOAcomponent, which are active in the k-th frame, and the sets

_(E)(k−1),

_(D)(k−1), and

_(U)(k−1) of coefficient indices of the ambient HOA component, whichhave to be enabled, disabled and to remain active in the (k 1)-th frame,are provided.

In the Predominant Sound Synthesis the HOA representation of thepredominant sound component Ĉ_(PS)(k−1) is computed from the frame{circumflex over (X)}_(PS) (k) of all predominant sound signals usingthe tuple set

_(DIR)(k+1) and the set ζ(k+1) of prediction parameters, the tuple set

_(VEC)(k+1) and the sets

_(E)(k−1),

_(D)(k−1), and

_(U)(k−1).

In the Ambience Synthesis, the ambient HOA component frame Ĉ_(AMB)(k−1)is created from the frame C_(I,AMB) (k) of the intermediaterepresentation of the ambient HOA component, using the set

_(AMB,ACT) (k) of indices of coefficient sequences of the ambient HOAcomponent which are active in the k-th frame. Note the delay of oneframe, which is introduced due to the synchronization with thepredominant sound HOA component. Finally, in the HOA Composition theambient HOA component frame Ĉ_(AMB)(k−1) and the frame Ĉ_(PS)(k−1) ofthe predominant sound HOA component are superposed to provide thedecoded HOA frame Ĉ(k−1).

As has become clear from the coarse description of the HOA compressionand decompression method above, the compressed representation consistsof I quantized monaural signals and some additional side information. Afixed number O_(MIN) out of these I quantized monaural signals representa spatially transformed version of the first O_(MIN) coefficientsequences of the ambient HOA component C_(AMB)(k−2). The type of theremaining I−O_(MIN) signals can vary between successive frame, beingeither directional, vector based, empty or representing an additionalcoefficient sequence of the ambient HOA component C_(AMB)(k−2). Taken asit is, the compressed HOA representation is meant to be monolithic. Inparticular, one problem is how to split the described representationinto a low quality base layer and an enhancement layer.

According to the disclosed invention, a candidate for a low quality baselayer are the O_(MIN) channels that contain a spatially transformedversion of the first O_(MIN) coefficient sequences of the ambient HOAcomponent C_(AMB)(k−2). What makes these (without loss of generality:first) O_(MIN) channels a good choice to form a low quality base layeris their time-invariant type. However, the respective signals lack anypredominant sound components, which are essential for the sound scene.This can also be seen in the computation of the ambient HOA componentC_(AMB)(k−1), which is carried out by subtraction of the predominantsound HOA representation C_(PS)(k−1) from the original HOArepresentation C(k−1) according to

C _(AMB)(k−1)=C(k−1)−C _(PS)(k−1)  (1)

A solution to this problem is to include the predominant soundcomponents at a low spatial resolution into the base layer.

Proposed amendments to the HOA compression are described in thefollowing.

FIG. 3 shows the structure of an architecture of a spatial HOA encodingand perceptual encoding portion of a HOA compressor according to oneembodiment of the invention. To include also the predominant soundcomponents at a low spatial resolution into the base layer, the ambientHOA component C_(AMB)(k−1), which is output by the HOA Decompositionprocessing in the spatial HOA encoder (see FIG. 1a ), is replaced by amodified version

$\begin{matrix}{{{\overset{\sim}{C}}_{AMB}( {k - 1} )} = \begin{bmatrix}{{\overset{\sim}{c}}_{{AMB},1}( {k - 1} )} \\{{\overset{\sim}{c}}_{{AMB},2}( {k - 1} )} \\\vdots \\{{\overset{\sim}{c}}_{{AMB},O}( {k - 1} )}\end{bmatrix}} & (2)\end{matrix}$

whose elements are given by

$\begin{matrix}{{{\overset{\sim}{c}}_{{AMB},n}( {k - 1} )} = \{ \begin{matrix}{c_{n}( {k - 1} )} & {{{for}\mspace{14mu} 1} \leq n \leq O_{MIN}} \\{c_{{AMB},n}( {k - 1} )} & {{{{for}\mspace{14mu} O_{MIN}} + 1} \leq n \leq 0}\end{matrix} } & (3)\end{matrix}$

In other words, the first O_(MIN) coefficient sequences of the ambientHOA component which are supposed to be always transmitted in a spatiallytransformed form, are replaced by the coefficient sequences of theoriginal HOA component. The other processing blocks of the spatial HOAencoder can remain unchanged.

It is important to note that this change of the HOA Decompositionprocessing can be seen as an initial operation making the HOAcompression work in a so-called “dual layer” or “two layer” mode. Thismode provides a bit stream that can be split up into a low quality BaseLayer and an Enhancement Layer. Using or not this mode can be signalizedby a single bit in access units of the total bit stream.

A possible consequent modification of the bit stream multiplexing toprovide bit streams for a base layer and an enhancement layer isillustrated in FIGS. 3 and 4, as described further below.

The base layer bit stream {hacek over (B)}_(BASE)(k−2) only includes theperceptually encoded signals {hacek over (z)}_(i)(k−2), i=1, . . . ,O_(MIN), and the corresponding coded gain control side information,consisting of the exponents e_(i)(k−2) and the exception flagsβ_(i)(k−2), i=1, . . . , O_(MIN). The remaining perceptually encodedsignals {hacek over (z)}₁(k−2), i=O_(MIN)+1, . . . , 0 and the encodedremaining side information are included into the enhancement layer bitstream. The base layer and enhancement layer bit streams {hacek over(B)}_(BASE)(k−2) and {hacek over (B)}_(ENH)(k−2) are then jointlytransmitted instead of the former total bit stream {hacek over(B)}(k−2).

In FIG. 3 and FIG. 4, an apparatus for compressing a HOA signal being aninput HOA representation with input time frames (C(k)) of HOAcoefficient sequences is shown. Said apparatus comprises a spatial HOAencoding and perceptual encoding portion for spatial HOA encoding of theinput time frames and subsequent perceptual encoding, which is shown inFIG. 3, and a source coder portion for source encoding, which is shownin FIG. 4.

The spatial HOA encoding and perceptual encoding portion comprises aDirection and Vector Estimation block 301, a HOA Decomposition block303, an Ambient Component Modification block 304, a Channel Assignmentblock 305, and a plurality of Gain Control blocks 306.

The Direction and Vector Estimation block 301 is adapted for performingDirection and Vector Estimation processing of the HOA signal, whereindata comprising first tuple sets

_(DIR)(k) for directional signals and second tuple sets

_(VEC)(k) for vector based signals are obtained, each of the first tuplesets

_(DIR)(k) comprising an index of a directional signal and a respectivequantized direction, and each of the second tuple sets

_(VEC)(k) comprising an index of a vector based signal and a vectordefining the directional distribution of the signals.

The HOA Decomposition block 303 is adapted for decomposing each inputtime frame of the HOA coefficient sequences into a frame of a pluralityof predominant sound signals X_(PS)(k−1) and a frame of an ambient HOAcomponent {tilde over (C)}_(AMB)(k−1), wherein the predominant soundsignals X_(PS)(k−1) comprise said directional sound signals and saidvector based sound signals, and wherein the ambient HOA component {tildeover (C)}_(AMB)(k−1) comprises HOA coefficient sequences representing aresidual between the input HOA representation and the HOA representationof the predominant sound signals, and wherein the decomposing furtherprovides prediction parameters ξ(k−1) and a target assignment vectorv_(A,T)(k−1). The prediction parameters ξ(k−1) describe how to predictportions of the HOA signal representation from the directional signalswithin the predominant sound signals X_(PS)(k−1) so as to enrichpredominant sound HOA components, and the target assignment vectorv_(A,T)(k−1) contains information about how to assign the predominantsound signals to a given number I of channels. The Ambient ComponentModification block 304 is adapted for modifying the ambient HOAcomponent C_(AMB)(k−1) according to the information provided by thetarget assignment vector v_(A,T)(k−1), wherein it is determined whichcoefficient sequences of the ambient HOA component C_(AMB)(k−1) are tobe transmitted in the given number I of channels, depending on how manychannels are occupied by predominant sound signals, and wherein amodified ambient HOA component C_(M,A)(k−2) and a temporally predictedmodified ambient HOA component C_(P,M,A)(k−1) are obtained, and whereina final assignment vector v_(A)(k−2) is obtained from information in thetarget assignment vector v_(A,T)(k−1).

The Channel Assignment block 305 is adapted for assigning thepredominant sound signals X_(PS)(k−1) obtained from the decomposing, thedetermined coefficient sequences of the modified ambient HOA componentC_(M,A)(k−2) and of the temporally predicted modified ambient HOAcomponent C_(P,M,A)(k−1) to the given number I of channels using theinformation provided by the final assignment vector v_(A)(k−2), whereintransport signals y_(i)(k−2), i=1, . . . ,I and predicted transportsignals y_(P,i)(k−2), i=1, . . . ,I are obtained.

The plurality of Gain Control blocks 306 is adapted for performing gaincontrol (805) to the transport signals y_(i)(k−2) and the predictedtransport signals y_(P,i)(k−2), wherein gain modified transport signalsz_(i)(k−2), exponents e_(i)(k−2) and exception flags β_(i)(k−2) areobtained.

FIG. 4 shows the structure of an architecture of a source coder portionof a HOA compressor according to one embodiment of the invention. Thesource coder portion as shown in FIG. 4 comprises a Perceptual Coder310, a Side Information Source Coder block with two coders 320,330,namely a Base Layer Side Information Source Coder 320 and an EnhancementLayer Side Information Encoder 330, and two multiplexers 340,350, namelya Base Layer Bitstream Multiplexer 340 and an Enhancement LayerBitstream Multiplexer 350. The Side Information Source Coders may be ina single Side Information Source Coder block.

The Perceptual Coder 310 is adapted for perceptually coding 806 saidgain modified transport signals z_(i)(k−2), wherein perceptually encodedtransport signals {hacek over (z)}_(L)(k−2), i=1, . . . , I areobtained.

The Side Information Source Coders 320,330 are adapted for encoding sideinformation comprising said exponents e_(i)(k−2) and exception flagsβ_(i)(k−2), said first tuple sets

_(DIR)(k) and second tuple sets

_(VEC)(k), said prediction parameters ξ(k−1) and said final assignmentvector v_(A)(k−2), wherein encoded side information {hacek over(Γ)}(k−2) is obtained. The multiplexers 340,350 are adapted formultiplexing the perceptually encoded transport signals {hacek over(z)}_(i)(k−2) and the encoded side information {hacek over (Γ)}(k−2)into a multiplexed data stream

(k−2), wherein the ambient HOA component {tilde over (C)}_(AMB)(k−1)obtained in the decomposing comprises first HOA coefficient sequences ofthe input HOA representation c_(n)(k−1) in O_(MIN) lowest positions (ie.those with lowest indices) and second HOA coefficient sequencesc_(AMB,n)(k−1) in remaining higher positions. As explained below withrespect to eq. (4)-(6), the second HOA coefficient sequences are part ofan HOA representation of a residual between the input HOA representationand the HOA representation of the predominant sound signals. Further,the first O_(MIN) exponents e_(i)(k−2), i=1, . . . , O_(MIN) andexception flags β_(i)(k−2), i=1, . . . , O_(MIN) are encoded in a BaseLayer Side Information Source Coder 320, wherein encoded Base Layer sideinformation {hacek over (Γ)}_(BASE)(k−2) is obtained, and whereinO_(MIN)=(N_(MIN)+1)² and O=(N+1)², with N_(MIN)≦N and O_(MIN)≦I andN_(MIN) is a predefined integer value. The first O_(MIN) perceptuallyencoded transport signals {hacek over (z)}_(i)(k−2), i=1, . . . ,O_(MIN) and the encoded Base Layer side information {hacek over(Γ)}_(BASE)(k−2) are multiplexed in a Base Layer Bitstream Multiplexer340 (which is one of said multiplexers), wherein a Base Layer bitstream{hacek over (B)}_(BASE)(k−2) is obtained. The Base Layer SideInformation Source Coder 320 is one of the Side Information SourceCoders, or it is within a Side Information Source Coder block. Theremaining I−O_(MIN) exponents e_(i)(k−2), i=O_(MIN)+1, . . . ,I andexception flags β_(i)(k−2), i=O_(MIN)+1, . . . , I, said first tuplesets

_(DIR)(k−1) and second tuple sets

_(VEC)(k−1), said prediction parameters ξ(k−1) and said final assignmentvector v_(A)(k−2) are encoded in an Enhancement Layer Side InformationEncoder 330, wherein encoded enhancement layer side information {hacekover (Γ)}_(ENH)(k−2) is obtained. The Enhancement Layer Side InformationSource Coder 330 is one of the Side Information Source Coders, or iswithin a Side Information Source Coder block.

The remaining I−O_(MIN) perceptually encoded transport signals {hacekover (z)}_(i)(k−2), i=O_(MIN)+1, . . . , I and the encoded enhancementlayer side information {hacek over (Γ)}_(ENH)(k−2) are multiplexed in anEnhancement Layer Bitstream Multiplexer 350 (which is also one of saidmultiplexers), wherein an Enhancement Layer bitstream {hacek over(B)}_(ENH)(k−2) is obtained. Further, a mode indication LMF_(E) is addedin a multiplexer or an indication insertion block. The mode indicationLMF_(E) signalizes usage of a layered mode, which is used for correctdecompression of the compressed signal.

In one embodiment, the apparatus for encoding further comprises a modeselector adapted for selecting a mode, the mode being indicated by themode indication LMF_(E) and being one of a layered mode and anon-layered mode. In the non-layered mode, the ambient HOA component{tilde over (C)}_(AMB)(k−1) comprises only HOA coefficient sequencesrepresenting a residual between the input HOA representation and the HOArepresentation of the predominant sound signals (ie., no coefficientsequences of the input HOA representation).

Proposed amendments of the HOA decompression are described in thefollowing.

In the layered mode, the modification of the ambient HOA componentC_(AMB)(k−1) in the HOA compression is considered at the HOAdecompression by appropriately modifying the HOA composition.

In the HOA decompressor, the demultiplexing and decoding of the baselayer and enhancement layer bit streams are performed according to FIG.5. The base layer bit stream {hacek over (B)}_(BASE) (k) isde-multiplexed into the coded representation of the base layer sideinformation and the perceptually encoded signals. Subsequently, thecoded representation of the base layer side information and theperceptually encoded signals are decoded to provide the exponentse_(i)(k) and the exception flags on the one hand, and the perceptuallydecoded signals on the other hand. Similarly, the enhancement layer bitstream is de-multiplexed and decoded to provide the perceptually decodedsignals and the remaining side information (see FIG. 5). With thislayered mode, the spatial HOA decoding part also has to be modified toconsider the modification of the ambient HOA component C_(AMB)(k−1) inthe spatial HOA encoding. The modification is accomplished in the HOAcomposition.

In particular, the reconstructed HOA representation

{circumflex over (C)}(k−1)=Ĉ _(PS)(k−1)+Ĉ _(AMB)(k−1)  (4)

is replaced by its modified version

$\begin{matrix}{{\overset{\sim}{\hat{C}}( {k - 1} )} = \begin{bmatrix}{{\overset{\sim}{\hat{c}}}_{1}( {k - 1} )} \\{{\overset{\sim}{\hat{c}}}_{2}( {k - 1} )} \\\vdots \\{{\overset{\sim}{\hat{c}}}_{O}( {k - 1} )}\end{bmatrix}} & (5)\end{matrix}$

whose elements are given by

$\begin{matrix}{{{\overset{\sim}{\hat{c}}}_{n}( {k - 1} )} = \{ \begin{matrix}{{\hat{c}}_{{AMB},n}( {k - 1} )} & {{{for}\mspace{14mu} 1} \leq n \leq O_{MIN}} \\{{\hat{c}}_{n}( {k - 1} )} & {{{{for}\mspace{14mu} O_{MIN}} + 1} \leq n \leq O}\end{matrix} } & (6)\end{matrix}$

That means that the predominant sound HOA component is not added to theambient HOA component for the first O_(MIN) coefficient sequences, sinceit is already included therein. All other processing blocks of the HOAspatial decoder remain unchanged.

In the following, the HOA decompression in the pure presence of a lowquality base layer bit stream {hacek over (B)}_(BASE)(k) is brieflyconsidered.

The bit stream is first de-multiplexed and decoded to provide thereconstructed signals {circumflex over (z)}_(i)(k) and the correspondinggain control side information, consisting of the exponents e_(i)(k) andthe exception flags β_(i)(k), i=1, . . . , O_(MIN). Note that in absenceof the enhancement layer, the perceptually coded signals {hacek over(z)}_(i)(k−2), i=O_(MIN)+1, . . . , O, are not available. A possible wayof addressing this situation is to set the signals {circumflex over(z)}_(i)(k), i=O_(MIN)+1, . . . , O, to zero, which automatically causesthe reconstructed predominant sound component C_(PS)(k−1) to be zero.

In a next step, in the spatial HOA decoder, the first O_(MIN) InverseGain Control processing blocks provide gain corrected signal framesŷ_(i)(k), i=1, . . . , O_(MIN), which are used to construct the frameC_(I,AMB) (k) of an intermediate representation of the ambient HOAcomponent by the Channel Reassignment. Note that the set

_(AMB,ACT)(k) of indices of coefficient sequences of the ambient HOAcomponent, which are active in the k-th frame, contains only the indices1,2, . . . , O_(MIN). In the Ambience Synthesis, the spatial transformof the first O_(MIN) coefficient sequences is reverted to provide theambient HOA component frame C_(AMB)(k−1). Finally, the reconstructed HOArepresentation is computed according to eq.(6).

FIG. 5 and FIG. 6 show the structure of an architecture of a HOAdecompressor according to one embodiment of the invention. The apparatuscomprises a perceptual decoding and source decoding portion as shown inFIG. 5, a spatial HOA decoding portion as shown in FIG. 6, and a modedetector adapted for detecting a layered mode indication LMF_(D)indicating that the compressed HOA signal comprises a compressed baselayer bitstream {hacek over (B)}_(BASE)(k) and a compressed enhancementlayer bitstream.

FIG. 5 shows the structure of an architecture of a perceptual decodingand source decoding portion of a HOA decompressor according to oneembodiment of the invention. The perceptual decoding and source decodingportion comprises a first demultiplexer 510, a second demultiplexer 520,a Base Layer Perceptual Decoder 540 and an Enhancement Layer PerceptualDecoder 550, a Base Layer Side Information Source Decoder 530 and anEnhancement Layer Side Information Source Decoder 560.

The first demultiplexer 510 is adapted for demultiplexing the compressedbase layer bitstream {hacek over (B)}_(BASE)(k), wherein firstperceptually encoded transport signals {hacek over (z)}_(i)(k), i=1, . .. , O_(MIN) and first encoded side information {hacek over(Γ)}_(BASE)(k) are obtained. The second demultiplexer 520 is adapted fordemultiplexing the compressed enhancement layer bitstream {hacek over(B)}_(ENH)(k), wherein second perceptually encoded transport signals{hacek over (z)}_(i)(k), i=O_(MIN)+1, . . . ,I and second encoded sideinformation {hacek over (Γ)}_(ENH)(k) are obtained.

The Base Layer Perceptual Decoder 540 and the Enhancement LayerPerceptual Decoder 550 are adapted for perceptually decoding 904 theperceptually encoded transport signals {hacek over (z)}_(i)(k), i=1, . .. , I, wherein perceptually decoded transport signals {hacek over(z)}_(i)(k) are obtained, and wherein in the Base Layer PerceptualDecoder 540 said first perceptually encoded transport signals {hacekover (z)}_(i)(k), i=1, . . . , O_(MIN) of the base layer are decoded andfirst perceptually decoded transport signals {circumflex over (z)}_(i)(k), i=1, . . . , O_(MIN) are obtained. In the Enhancement LayerPerceptual Decoder 550, said second perceptually encoded transportsignals {hacek over (z)}_(i)(k), i=O_(MIN)+1, . . . , I of theenhancement layer are decoded and second perceptually decoded transportsignals {circumflex over (z)}_(i) (k), i=O_(MIN)+1, . . . , I areobtained.

The Base Layer Side Information Source Decoder 530 is adapted fordecoding 905 the first encoded side information {hacek over(Γ)}_(BASE)(k), wherein first exponents e_(i)(k), i=1, . . . , O_(MIN)and first exception flags β_(i)(k), i=1, . . . , O_(MIN) are obtained.

The Enhancement Layer Side Information Source Decoder 560 is adapted fordecoding 906 the second encoded side information {hacek over(Γ)}_(ENH)(k), wherein second exponents e_(i)(k), i=O_(MIN)+1, . . . , Iand second exception flags β_(i)(k), i=O_(MIN)+1, . . . , I areobtained, and wherein further data are obtained. The further datacomprise a first tuple set

_(DIR)(k+1) for directional signals and a second tuple set

_(VEC)(k+1) for vector based signals. Each tuple of the first tuple set

_(DIR)(k+1) comprises an index of a directional signal and a respectivequantized direction, and each tuple of the second tuple set

_(VEC)(k+1) comprises an index of a vector based signal and a vectordefining the directional distribution of the vector based signal.Further, prediction parameters ξ(k+1) and an ambient assignment vectorv_(AMB,ASSIGN)(k) are obtained, wherein the ambient assignment vectorv_(AMS,ASSIGN)(k) comprises components that indicate for eachtransmission channel if and which coefficient sequence of the ambientHOA component it contains.

FIG. 6 shows the structure of an architecture of a spatial HOA decodingportion of a HOA decompressor according to one embodiment of theinvention. The spatial HOA decoding portion comprises a plurality ofinverse gain control units 604, a Channel Reassignment block 605, aPredominant Sound Synthesis block 606, and an Ambient Synthesis block607, a HOA Composition block 608.

The plurality of inverse gain control units 604 are adapted forperforming inverse gain control, wherein said first perceptually decodedtransport signals {circumflex over (z)}_(i)(k), i=1, . . . O_(MIN) aretransformed into first gain corrected signal frames ŷ_(i)(k), i=1, . . ., O_(MIN) according to the first exponents e_(i)(k), i=1, . . . ,O_(MIN) and the first exception flags β_(i)(k), i=1, . . . , O_(MIN),and wherein the second perceptually decoded transport signals{circumflex over (z)}_(i) (k), i=O_(MIN)+1, . . . I are transformed intosecond gain corrected signal frames ŷ_(i)(k), i=O_(MIN)+1, . . . ,Iaccording to the second exponents e_(i)(k),i=O_(MIN)+1, . . . , I andthe second exception flags β_(i)(k), i=O_(MIN)+1, . . . , I.

The Channel Reassignment block 605 is adapted for redistributing 911 thefirst and second gain corrected signal frames ŷ_(i)(k), i=1, . . . , Ito I channels, wherein frames of predominant sound signals {circumflexover (X)}_(PS)(k) are reconstructed, the predominant sound signalscomprising directional signals and vector based signals, and wherein amodified ambient HOA component {tilde over (C)}_(I,AMB)(k) is obtained,and wherein the assigning is made according to said ambient assignmentvector v_(AMB,ASSIGN)(k) and to information in said first and secondtuple sets

_(DIR)(k+1),

_(VEC)(k+1).

Further, the Channel Reassignment block 605 is adapted for generating afirst set of indices

_(AMB,ACT)(k) of coefficient sequences of the modified ambient HOAcomponent that are active in a k^(th) frame, and a second set of indices

_(E)(k−1),

_(D)(k−1),

_(U)(k−1) of coefficient sequences of the modified ambient HOA componentthat have to be enabled, disabled and to remain active in the (k−1)^(th)frame.

The Predominant Sound Synthesis block 606 is adapted for synthesizing912 a HOA representation of the predominant HOA sound componentsĈ_(PS)(k−1) from said predominant sound signals {circumflex over(X)}_(PS)(k), wherein the first and second tuple sets

_(DIB)(k+1),

_(VEC)(k+1), the prediction parameters ξ(k+1) and the second set ofindices

_(E)(k

_(D)(k−1),

_(U)(k−1) are used.

The Ambient Synthesis block 607 is adapted for synthesizing 913 anambient HOA component

_(AME) (k−1) from the modified ambient HOA component {tilde over(C)}_(I,AMB) (k) wherein an inverse spatial transform for the firstO_(m) channels is made and wherein the first set of indices

_(AMB,ACT) (k) is used, the first set of indices being indices ofcoefficient sequences of the ambient HOA component that are active inthe k^(th) frame.

If the layered mode indication LMF_(D) indicates a layered mode with atleast two layers, the ambient HOA component comprises in its O_(MIN)lowest positions (ie. those with lowest indices) HOA coefficientsequences of the decompressed HOA signal Ĉ(k−1), and in remaining higherpositions coefficient sequences that are part of an HOA representationof a residual. This residual is a residual between the decompressed HOAsignal Ĉ(k−1) and 914 the HOA representation of the predominant HOAsound components Ĉ_(PS)(k−1).

On the other hand, if the layered mode indication LMF_(D) indicates asingle-layer mode, there are no HOA coefficient sequences of thedecompressed HOA signal Ĉ(k−1) comprised, and the ambient HOA componentis a residual between the decompressed HOA signal Ĉ(k−1) and the HOArepresentation of the predominant sound components Ĉ_(PS)(k−1).

The HOA Composition block 608 is adapted for adding the HOArepresentation of the predominant sound components to the ambient HOAcomponent Ĉ_(PS)(k−1)

_(AMB)(k−1), wherein coefficients of the HOA representation of thepredominant sound signals and corresponding coefficients of the ambientHOA component are added, and wherein the decompressed HOA signal Ĉ′(k−1)is obtained, and wherein, if the layered mode indication LMF_(D)indicates a layered mode with at least two layers, only the highestI−O_(MIN) coefficient channels are obtained by addition of thepredominant HOA sound components Ĉ_(PS)(k−1) and the ambient HOAcomponent

_(AMB)(k−1), and the lowest O_(MIN) coefficient channels of thedecompressed HOA signal Ĉ′(k−1) are copied from the ambient HOAcomponent

_(AMB)(k−1). On the other hand, if the layered mode indication LMF_(D)indicates a single-layer mode, all coefficient channels of thedecompressed HOA signal Ĉ′(k−1) are obtained by addition of thepredominant HOA sound components Ĉ_(PS)(k−1) and the ambient HOAcomponent

_(AMB)(k−1).

FIG. 7 shows transformation of frames from ambient HOA signals tomodified ambient HOA signals.

FIG. 8 shows a flow-chart of a method for compressing a HOA signal.

The method 800 for compressing a Higher Order Ambisonics (HOA) signalbeing an input HOA representation of an order N with input time framesC(k) of HOA coefficient sequences comprises spatial HOA encoding of theinput time frames and subsequent perceptual encoding and sourceencoding.

The spatial HOA encoding comprises steps of

performing Direction and Vector Estimation processing 801 of the HOAsignal in a Direction and Vector Estimation block 301, wherein datacomprising first tuple sets

_(DIR)(k) for directional signals and second tuple sets

_(VEC)(k) for vector based signals are obtained, each of the first tuplesets

_(DIR)(k) comprising an index of a directional signal and a respectivequantized direction, and each of the second tuple sets

_(VEC)(k) comprising an index of a vector based signal and a vectordefining the directional distribution of the signals,

decomposing 802 in a HOA Decomposition block 303 each input time frameof the HOA coefficient sequences into a frame of a plurality ofpredominant sound signals X_(PS)(k−1) and a frame of an ambient HOAcomponent {tilde over (C)}_(AMB)(k−1), wherein the predominant soundsignals X_(PS)(k−1) comprise said directional sound signals and saidvector based sound signals, and wherein the ambient HOA component {tildeover (C)}_(AMB)(k−1) comprises HOA coefficient sequences representing aresidual between the input HOA representation and the HOA representationof the predominant sound signals, and wherein the decomposing 702further provides prediction parameters ξ(k−1) and a target assignmentvector v_(A,T)(k−1), the prediction parameters ξ(k−1) describing how topredict portions of the HOA signal representation from the directionalsignals within the predominant sound signals X_(PS)(k−1) so as to enrichpredominant sound HOA components, and the target assignment vectorv_(A,T)(k−1) containing information about how to assign the predominantsound signals to a given number I of channels,

modifying 803 in an Ambient Component Modification block 304 the ambientHOA component C_(AMB)(k−1) according to the information provided by thetarget assignment vector v_(A,T)(k−1), wherein it is determined whichcoefficient sequences of the ambient HOA component C_(AMB)(k−1) are tobe transmitted in the given number I of channels, depending on how manychannels are occupied by predominant sound signals, and wherein amodified ambient HOA component C_(M,A)(k−2) and a temporally predictedmodified ambient HOA component C_(P,M,A)(k−1) are obtained, and whereina final assignment vector v_(A)(k−2) is obtained from information in thetarget assignment vector v_(A,T)(k−1),

assigning 804 in a Channel Assignment block 105 the predominant soundsignals X_(PS)(k−1) obtained from the decomposing, and the determinedcoefficient sequences of the modified ambient HOA component C_(M,A)(k−2)and of the temporally predicted modified ambient HOA componentC_(P,M,A)(k−1) to the given number I of channels using the informationprovided by the final assignment vector v_(A)(k−2), wherein transportsignals y_(i)(k−2), i=1, . . . , I and predicted transport signalsy_(P,i)(k−2), i=1, . . . , I are obtained, and performing gain control805 to the transport signals y_(i)(k−2) and the predicted transportsignals y_(P,i)(k−2) in a plurality of Gain Control blocks 306, whereingain modified transport signals z_(i)(k−2), exponents e_(i)(k−2) andexception flags β_(i)(k−2) are obtained.

The perceptual encoding and source encoding comprises steps ofperceptually coding 806 in a Perceptual Coder 310 said gain modifiedtransport signals z_(i)(k−2), wherein perceptually encoded transportsignals {hacek over (z)}_(i)(k−2), i=1, . . . , I are obtained,

encoding 807 in one or more Side Information Source Coders 320,330 sideinformation comprising said exponents e_(i)(k−2) and exception flagsβ_(i)(k−2), said first tuple sets

_(DIR)(k) and second tuple sets

_(VEC)(k), said prediction parameters ξ(k−1) and said final assignmentvector v_(A)(k−2), wherein encoded side information {hacek over(Γ)}(k−2) is obtained;

and

multiplexing 808 the perceptually encoded transport signals {hacek over(z)}_(i)(k−2) and the encoded side information {hacek over (Γ)}(k−2),wherein a multiplexed data stream

(k−2) is obtained. The ambient HOA component {tilde over (C)}_(AMB)(k−1)obtained in the decomposing step 802 comprises first HOA coefficientsequences of the input HOA representation c_(n)(k−1) in O_(MIN) lowestpositions (ie. those with lowest indices) and second HOA coefficientsequences c_(AMB,n)(k−1) in remaining higher positions. The secondcoefficient sequences are part of an HOA representation of a residualbetween the input HOA representation and the HOA representation of thepredominant sound signals.

The first O_(MIN) exponents e_(i)(k−2), i=1, . . . , O_(MIN) andexception flags β_(i)(k−2), i=1, . . . , O_(MIN) are encoded in a BaseLayer Side Information Source Coder 320, wherein encoded Base Layer sideinformation {hacek over (Γ)}_(BASE)(k−2) is obtained, and whereinO_(MIN)=(N_(MIN)+1)² and O=(N+1)², with N_(MIN)≦N and O_(MIN)≦I andN_(MIN) is a predefined integer value.

The first O_(MIN) perceptually encoded transport signals {hacek over(z)}_(i)(k−2), i=1, . . . , O_(MIN) and the encoded Base Layer sideinformation {hacek over (Γ)}_(BASE)(k−2) are multiplexed 809 in a BaseLayer Bitstream Multiplexer 340, wherein a Base Layer bitstream {hacekover (B)}_(BASE)(k−2) is obtained. The remaining I−O_(MIN) exponentse_(i)(k−2), i=O_(MIN)+1, . . . , I and exception flags β_(i)(k−2),i=O_(MIN)+1, . . . , I, said first tuple sets

_(DIR)(k−1) and second tuple sets

_(VEC)(k−1), said prediction parameters ξ(k−1) and said final assignmentvector v_(A)(k−2) (also shown as v_(AMB,ASSIGN)(k) in the Figures) areencoded in an Enhancement Layer Side Information Encoder 330, whereinencoded enhancement layer side information {hacek over (Γ)}_(ENH)(k−2)is obtained.

The remaining I−O_(MIN) perceptually encoded transport signals {hacekover (z)}_(i)(k−2), i=O_(MIN)+1, . . . , I and the encoded enhancementlayer side information {hacek over (Γ)}_(ENH)(k−2) are multiplexed 810in an Enhancement Layer Bitstream Multiplexer 350, wherein anEnhancement Layer bitstream {hacek over (B)}_(ENH)(k−2) is obtained.

A mode indication is added 811 that signalizes usage of a layered mode,as described above. The mode indication is added by an indicationinsertion block or a multiplexer.

In one embodiment, the method further comprises a final step ofmultiplexing the Base Layer bitstream {hacek over (B)}_(BASE)(k−2),Enhancement Layer bitstream {hacek over (B)}_(ENH)(k−2) and modeindication into a single bitstream.

In one embodiment, said dominant direction estimation is dependent on adirectional power distribution of the energetically dominant HOAcomponents.

In one embodiment, in modifying the ambient HOA component, a fade in andfade out of coefficient sequences is performed if the HOA sequenceindices of the chosen HOA coefficient sequences vary between successiveframes.

In one embodiment, in modifying the ambient HOA component, a partialdecorrelation of the ambient HOA component C_(AMB)(k−1) is performed.

In one embodiment, quantized direction comprised in the first tuple sets

_(DIR)(k) is a dominant direction.

FIG. 9 shows a flow-chart of a method for decompressing a compressed HOAsignal. In this embodiment of the invention, the method 900 fordecompressing a compressed HOA signal comprises perceptual decoding andsource decoding and subsequent spatial HOA decoding to obtain outputtime frames Ĉ(k−1) of HOA coefficient sequences, and the methodcomprises a step of detecting 901 a layered mode indication LMF_(D)indicating that the compressed Higher Order Ambisonics (HOA) signalcomprises a compressed base layer bitstream {hacek over (B)}_(BASE)(k)and a compressed enhancement layer bitstream {hacek over (B)}_(ENH)(k).

The perceptual decoding and source decoding comprises steps of

demultiplexing 902 the compressed base layer bitstream {hacek over(B)}_(BASE)(k), wherein first perceptually encoded transport signals{hacek over (z)}₁(k), i=1, . . . , O_(MIN) and first encoded sideinformation {hacek over (Γ)}_(EASE) (k) are obtained,

demultiplexing 903 the compressed enhancement layer bitstream {hacekover (B)}_(ENH)(k), wherein second perceptually encoded transportsignals {hacek over (z)}_(i)(k), i=O_(MIN)+1, . . . ,I and secondencoded side information {hacek over (Γ)}_(ENH)(k) are obtained,

perceptually decoding 904 the perceptually encoded transport signals{hacek over (z)}_(i)(k), i=1, . . . ,I, wherein perceptually decodedtransport signals {circumflex over (z)}_(i) (k) are obtained, andwherein in a Base Layer Perceptual Decoder 540 said first perceptuallyencoded transport signals {hacek over (z)}_(i)(k), i=1, . . . , O_(MIN)of the base layer are decoded and first perceptually decoded transportsignals {circumflex over (z)}_(i) (k), i=1, . . . , O_(MIN) areobtained, and wherein in an Enhancement Layer Perceptual Decoder 550said second perceptually encoded transport signals {hacek over(z)}_(i)(k), i=O_(MIN)+1, . . . , I of the enhancement layer are decodedand second perceptually

decoded transport signals {circumflex over (z)}_(i)(k), i=O_(MIN)+1, . .. , I are obtained,

decoding 905 the first encoded side information {hacek over (Γ)}_(gAsE)(k) in a Base Layer Side Information Source Decoder 530, wherein firstexponents e_(i)(k),i=1, . . . , O_(MIN) and first exception flagsβ_(i)(k), i=1, . . . , O_(MIN) are obtained, and

decoding 906 the second encoded side information {hacek over(Γ)}_(ENH)(k) in an Enhancement Layer Side Information Source Decoder560, wherein second exponents e_(i)(k), i=O_(MIN)+1, . . . , I andsecond exception flags β_(i)(k), i=O_(MIN)+1, . . . , I are obtained,and wherein further data are obtained, the further data comprising afirst tuple set

_(DIR)(k+1) for directional signals and a second tuple set

_(VEC)(k+1) for vector based signals, each tuple of the first tuple set

_(DIR)(k+1) comprising an index of a directional signal and a respectivequantized direction, and each tuple of the second tuple set

_(VEC)(k+1) comprising an index of a vector based signal and a vectordefining the directional distribution of the vector based signal, andfurther wherein prediction parameters ξ(k+1) and an ambient assignmentvector v_(AMB,ASSIGN) (k) are obtained. The ambient assignment vectorv_(AMB,ASSIGN)(k) comprises components that indicate for eachtransmission channel if and which coefficient sequence of the ambientHOA component it contains.

The spatial HOA decoding comprises steps of

performing 910 inverse gain control, wherein said first perceptuallydecoded transport signals {circumflex over (z)}_(i)(k), i=1, . . . ,O_(MIN) are transformed into first gain corrected signal framesŷ_(i)(k), i=1, . . . , O_(MIN) according to said first exponentse_(i)(k),i=1, . . . , O_(MIN) and said first exception flags β_(i)(k),i=1, . . . , O_(MIN), and wherein said second perceptually decodedtransport signals {circumflex over (z)}_(i) (k), i=O_(MIN)+1, . . . , Iare transformed into second gain corrected signal frames ŷ_(i)(k),i=O_(MIN)+1, . . . , I according to said second exponentse_(i)(k),i=O_(MIN)+1, . . . , I and said second exception flags(β_(i)(k), i=O_(MIN)+1, . . . , I, redistributing 911 in a ChannelReassignment block 605 the first and second gain corrected signal framesŷ_(i)(k), i=1, . . . , I to I channels, wherein frames of predominantsound signals {circumflex over (X)}_(PS)(k) are reconstructed, thepredominant sound signals comprising directional signals and vectorbased signals, and wherein a modified ambient HOA component {tilde over(C)}_(I,AMB)(k) is obtained, and wherein the assigning is made accordingto said ambient assignment vector v_(AMB,ASSIGN)(k) and to informationin said first and second tuple sets

_(DIR)(k+1),

_(VEC)(k+1),

generating 911 b in the Channel Reassignment block 605 a first set ofindices

_(AMB,ACT)(k) of coefficient sequences of the modified ambient HOAcomponent that are active in the k^(th) frame, and a second set ofindices

_(E)(k−1),

_(D)(k−1),

_(U)(k−1) of coefficient sequences of the modified ambient HOA componentthat have to be enabled, disabled and to remain active in the (k−1)^(th)frame,

synthesizing 912 in the Predominant Sound Synthesis block 606 a HOArepresentation of the predominant HOA sound components Ĉ_(PS)(k−1) fromsaid predominant sound signals {circumflex over (X)}_(PS) (k), whereinthe first and second tuple sets

_(DIR)(k+1),

_(VEC)(k+1)), the prediction parameters ξ(k+1) and the second set ofindices

_(E)(k−1),

_(D)(k−1),

_(U)(k−1) are used,

synthesizing 913 in the Ambient Synthesis block 607 an ambient HOAcomponent

_(AMB)(k−1) from the modified ambient HOA component {tilde over(C)}_(I,AMB)(k), wherein an inverse spatial transform for the firstO_(m) channels is made and wherein the first set of indices

_(AMB,ACT)(k) is used, the first set of indices being indices ofcoefficient sequences of the ambient HOA component that are active inthe k^(th) frame, wherein the ambient HOA component has one of at leasttwo different configurations, depending on the layered mode indicationLMF_(D), and

adding 914 the HOA representation of the predominant HOA soundcomponents Ĉ_(PS)(k−1) and the ambient HOA component

_(AMB)(k−1) in a HOA Composition block 608, wherein coefficients of theHOA representation of the predominant sound signals and correspondingcoefficients of the ambient HOA component are added, and wherein thedecompressed HOA signal Ĉ(k−1) is obtained, and wherein the followingconditions apply:

if the layered mode indication LMF_(D) indicates a layered mode with atleast two layers, only the highest I−O_(MIN) coefficient channels areobtained by addition of the predominant HOA sound components Ĉ_(PS)(k−1)and the ambient HOA component

_(AMB)(k−1), and the lowest O_(MIN) coefficient channels of thedecompressed HOA signal Ĉ(k−1) are copied from the ambient HOA component

_(AMB)(k−1). Otherwise, if the layered mode indication LMF_(D) indicatesa single-layer mode, all coefficient channels of the decompressed HOAsignal Ĉ(k−1) are obtained by addition of the predominant HOA soundcomponents Ĉ_(PS)(k−1) and the ambient HOA component

_(AMB)(k−1).

The configuration of the ambient HOA component in dependence of thelayered mode indication LMF_(D) is as follows:

If the layered mode indication LMF_(D) indicates a layered mode with atleast two layers, the ambient HOA component comprises in its O_(MIN)lowest positions HOA coefficient sequences of the decompressed HOAsignal Ĉ(k−1), and in remaining higher positions coefficient sequencesbeing part of an HOA representation of a residual between thedecompressed HOA signal Ĉ(k−1) and the HOA representation of thepredominant HOA sound components Ĉ_(PS)(k−1).

On the other hand, if the layered mode indication LMF_(D) indicates asingle-layer mode, the ambient HOA component is a residual between thedecompressed HOA signal Ĉ(k−1) and the HOA representation of thepredominant HOA sound components Ĉ_(PS)(k−1).

In one embodiment, the compressed HOA signal representation is in amultiplexed bitstream, and the method for decompressing the compressedHOA signal further comprises an initial step of demultiplexing thecompressed HOA signal representation, wherein said compressed base layerbitstream {hacek over (B)}_(BASE) (k), said compressed enhancement layerbitstream {hacek over (B)}_(ENH)(k) and said layered mode indicationLMF_(D) are obtained.

FIG. 10 shows details of parts of an architecture of a spatial HOAdecoding portion of a HOA decompressor according to one embodiment ofthe invention.

Advantageously, it is possible to decode only the BL, e.g. if no EL isreceived or if the BL quality is sufficient. For this case, signals ofthe EL can be set to zero at the decoder. Then, the redistributing 911the first and second gain corrected signal frames ŷ_(i)(k), i=1, . . . ,I to I channels in the Channel Reassignment block 605 is very simple,since the frames of predominant sound signals {circumflex over(X)}_(PS)(k) are empty. The second set of indices

_(E)(k−1),

_(D)(k−1),

_(U)(k−1) of coefficient sequences of the modified ambient HOA componentthat have to be enabled, disabled and to remain active in the (k−1)^(th)frame are set to zero. The synthesizing 912 the HOA representation ofthe predominant HOA sound components Ĉ_(PS)(k−1) from the predominantsound signals {circumflex over (X)}_(PS)(k) in the Predominant SoundSynthesis block 606 can therefore be skipped, and the synthesizing 913an ambient HOA component

_(AMB)(k−1) from the modified ambient HOA component {tilde over(C)}_(I,AMB)(k) in the Ambient Synthesis block 607 corresponds to aconventional HOA synthesis.

The original (ie. monolithic, non-scalable, non-layered) mode for theHOA compression may still be useful for applications where a low qualitybase layer bit stream is not required, e.g. for file based compression.A major advantage of perceptually coding the spatially transformed firstO_(MIN) coefficient sequences of the ambient HOA component C_(AMB),which is a difference between the original and the directional HOArepresentation, instead of the spatially transformed coefficientsequences of the original HOA component C, is that in the former casethe cross correlations between all signals to be perceptually coded arereduced. Any cross correlations between the signals z_(i), i=1, . . . ,I may cause a constructive superposition of the perceptual coding noiseduring the spatial decoding process, while at the same time thenoise-free HOA coefficient sequences are canceled at superposition. Thisphenomenon is known as perceptual noise unmasking. In the layered mode,there are high cross correlations between each of the signals z_(i),i=1, . . . , O_(MIN) and also between the signals z_(i), i=1, . . . ,O_(MIN) and z_(i), i=O_(MIN)+1, . . . , I, because the modifiedcoefficient sequences of the ambient HOA component {tilde over(c)}_(AMB,n), n=1, . . . , O_(MIN) include signals of the directionalHOA component (see eq.(3)). To the contrary, this is not the case forthe original, non-layered mode. It can therefore be concluded that thetransmission robustness introduced by the layered mode may come at theexpense of compression quality. However, the reduction in compressionquality is low compared to the increase in transmission robustness. Ashas been shown above, the proposed layered mode is advantageous in atleast the situations described above.

While there has been shown, described, and pointed out fundamental novelfeatures of the present invention as applied to preferred embodimentsthereof, it will be understood that various omissions and substitutionsand changes in the apparatus and method described, in the form anddetails of the devices disclosed, and in their operation, may be made bythose skilled in the art without departing from the spirit of thepresent invention. It is expressly intended that all combinations ofthose elements that perform substantially the same function insubstantially the same way to achieve the same results are within thescope of the invention. Substitutions of elements from one describedembodiment to another are also fully intended and contemplated.

It will be understood that the present invention has been describedpurely by way of example, and modifications of detail can be madewithout departing from the scope of the invention.

Each feature disclosed in the description and (where appropriate) theclaims and drawings may be provided independently or in any appropriatecombination. Features may, where appropriate be implemented in hardware,software, or a combination of the two. Connections may, whereapplicable, be implemented as wireless connections or wired, notnecessarily direct or dedicated, connections.

Reference numerals appearing in the claims are by way of illustrationonly and shall have no limiting effect on the scope of the claims.

CITED REFERENCES

-   [1] EP12306569.0-   [2] EP12305537.8 (published as EP2665208A)-   [3] EP133005558.2-   [4] ISO/IEC JTC1/SC29/WG11 N14264. Working draft 1-HOA text of    MPEG-H 3D audio, January 2014

1-6. (canceled)
 7. A method of decoding a compressed Higher OrderAmbisonics (HOA) representation of a sound or soundfield, the methodcomprising: receiving a bit stream containing the compressed HOArepresentation; decoding, based on a determination that there aremultiple layers, the compressed HOA representation from the bitstream toobtain a sequence of decoded HOA representations, wherein a first subsetof the sequence of decoded HOA representations is determined based onlyon corresponding ambient HOA components, and wherein a second subset ofthe sequence of decoded HOA representations is determined based oncorresponding ambient HOA components and corresponding predominant soundcomponents.
 8. The method of claim 7, wherein, for a frame k, thesequence of decoded HOA representations are represented at least in partby${{\overset{\sim}{\hat{c}}}_{n}( {k - 1} )} = \{ \begin{matrix}{{\overset{\sim}{\hat{c}}}_{{AMB},n}( {k - 1} )} & {{for}\mspace{14mu} n\mspace{14mu} {in}\mspace{14mu} a\mspace{14mu} {first}{\mspace{11mu} \;}{set}\mspace{14mu} {of}\mspace{14mu} {indices}} & \; \\\begin{matrix}{{{\hat{c}}_{n}( {k - 1} )} = {{{\hat{c}}_{{PS},n}( {k - 1} )} +}} \\{{{\hat{c}}_{{AMB},n}( {k - 1} )},}\end{matrix} & {{for}\mspace{14mu} n\mspace{14mu} {in}{\mspace{11mu} \;}a\mspace{14mu} {second}\mspace{14mu} {set}\mspace{14mu} {of}\mspace{14mu} {indices}} & \;\end{matrix} $ wherein ĉ_(AMB,n) (k−1) corresponds to the ambientHOA components and ĉ_(PS,n)(k−1) corresponds to the predominant soundcomponents.
 9. The method of claim 8, wherein the first set of indicesis determined based on 1≦n≦O_(MIN) and the second set of indices isdetermined based on O_(MIN)+1≦m≦O, wherein O indicates a total number ofchannels and O_(MIN) indicates a number between 1 and O.
 10. The methodof claim 9, wherein O_(MIN)=(N_(MIN)+1)² with N_(MIN)≦N, wherein N is anorder of input frames of the compressed HOA representation.
 11. Themethod of claim 7, wherein an indication of multiple layers is signalledin the bitstream.
 12. The method of claim 7, wherein the multiple layersinclude a base layer and at least an enhancement layer.
 13. The methodof claim 7, wherein, for a frame k, the sequence of decoded HOArepresentations is determined based on an ambient assignment vector(v_(AMB,ASSIGN)(k)) and a first tuple set

_(DIR)(k+1), comprising an index of a directional representation and arespective quantized direction and a second tuple set

_(VEC)(k+1)) comprising an index of a vector based representation and avector defining the directional distribution of the vector basedrepresentation.
 14. The method of claim 7, further comprisinggenerating, during channel reassignment, a third set of indices (

_(AMB,ACT)(k)) of coefficient sequences that are active in frame k, anda second set of indices (

_(E)(k−1),

_(D)(k−1),

_(U)(k−1)) of coefficient sequences of that have to be enabled, disabledand to remain active, respectively, in a frame (k−1).
 15. The method ofclaim 7, further determining, based on a determination that there arenot multiple layers, that there is a single layer, and, based on thedetermination of the single layer, determining, for a frame k, a singlelayer decoded HOA representation based on an addition of a correspondingpredominant HOA sound component (Ĉ_(PS)(k−1)) and a correspondingambient HOA component (

_(AMB)(k−1)).
 16. An apparatus for decoding a compressed Higher OrderAmbisonics (HOA) representation of a sound or a soundfield, theapparatus comprising: a receiver for receiving a bit stream containingthe compressed HOA representation; an audio decoder for decoding, basedon a determination that there are multiple layers, the compressed HOArepresentation from the bitstream to obtain a sequence of decoded HOArepresentations, wherein a first subset of the sequence of decoded HOArepresentations is determined based only on corresponding ambient HOAcomponents, and wherein a second subset of the sequence of decoded HOArepresentations is determined based on corresponding ambient HOAcomponents and corresponding predominant sound components.
 17. Theapparatus of claim 16, wherein, for a frame k, the sequence of decodedHOA representations are represented at least in part by${{\overset{\sim}{\hat{c}}}_{n}( {k - 1} )} = \{ \begin{matrix}{{\hat{c}}_{{AMB},n}( {k - 1} )} & {{for}\mspace{14mu} n\mspace{14mu} {in}\mspace{14mu} a\mspace{14mu} {first}{\mspace{11mu} \;}{set}\mspace{14mu} {of}\mspace{14mu} {indices}} & \; \\\begin{matrix}{{{\hat{c}}_{n}( {k - 1} )} = {{{\hat{c}}_{{PS},n}( {k - 1} )} +}} \\{{{\hat{c}}_{{AMB},n}( {k - 1} )},}\end{matrix} & {{for}\mspace{14mu} n\mspace{14mu} {in}{\mspace{11mu} \;}a\mspace{14mu} {second}\mspace{14mu} {set}\mspace{14mu} {of}\mspace{14mu} {indices}} & \;\end{matrix} $ wherein ĉ_(AMB,n)(k−1) corresponds to the ambientHOA components and ĉ_(PS,n)(k−1) corresponds to the predominant soundcomponents.
 18. The apparatus of claim 17, wherein the first set ofindices is determined based on 1≦n≦O_(MIN) and the second set of indicesis determined based on O_(MIN)+1≦m≦O, wherein O indicates a total numberof channels and O_(MIN) indicates a number between 1 and O.
 19. Theapparatus of claim 18, wherein O_(MIN)=(N_(MIN)+1)² with N_(MIN)≦N,wherein N is an order of input frames of the compressed HOArepresentation.
 20. The apparatus of claim 16, wherein an indication ofmultiple layers is signalled in the bitstream.
 21. The apparatus ofclaim 16, wherein the multiple layers include a base layer and at leastan enhancement layer.
 22. The apparatus of claim 16, wherein the audiodecoder is further configured to determine, for a frame k, the sequenceof decoded HOA representations based on an ambient assignment vector(v_(AMB,ASSIGN) (k)) and a first tuple set

_(DIR)(k+1), comprising an index of a directional representation and arespective quantized direction and a second tuple set

_(VEC)(k+1)) comprising an index of a vector based representation and avector defining the directional distribution of the vector basedrepresentation.
 23. The apparatus of claim 16, wherein the audio decoderis further configured to generate, during channel reassignment, a thirdset of indices (

_(AMB,ACT)(k)) of coefficient sequences that are active in frame k, anda second set of indices (

_(E)(k−1),

_(D)(k−1),

_(U)(k−1)) of coefficient sequences of that have to be enabled, disabledand to remain active, respectively, in a frame (k−1).
 24. The apparatusof claim 16, wherein the audio decoder is further configured todetermine, based on a determination that there are not multiple layers,that there is a single layer, and, based on the determination of thesingle layer, determining a single layer decoded HOA representationbased on an addition of a corresponding predominant HOA sound component(Ĉ_(PS)(k−1)) and a corresponding ambient HOA component (

_(AMB)(k−1)).
 25. A non-transitory computer readable storage mediumcontaining instructions that when executed by a processor perform amethod of decoding a compressed Higher Order Ambisonics (HOA)representation of a sound or soundfield, comprising: receiving a bitstream containing the compressed HOA representation; decoding, based ona determination that there are multiple layers, the compressed HOArepresentation from the bitstream to obtain a sequence of decoded HOArepresentations, wherein a first subset of the sequence of decoded HOArepresentations is determined based only on corresponding ambient HOAcomponents, and wherein a second subset of the sequence of decoded HOArepresentations is determined based on corresponding ambient HOAcomponents and corresponding predominant sound components.